Unreachable code

MSVC (VS 2022) reports ``` fast_float.h(462,1): warning C4702: unreachable code fast_float.h(518,1): warning C4702: unreachable code ``` for `fastfloat_strncasecmp3` and `fastfloat_strncasecmp5` on fast_float.h of v8.2.3. Note that it is compiled with `/std:c++20`.